1. Technical Field of the Invention
This invention pertains to frequency modulation (FM) radio receivers and in particular to FM receivers that use automatic frequency control (AFC) or phase lock to control a local oscillator.
2. Description of the Prior Art
Present two-way FM radio communications systems, with closely spaced channels at ultra-high frequencies (UHF), place stringent requirements on radio receivers. One requirement is for stable, accurate frequency determining elements; another is for narrow selectivity. To meet these requirements, most receivers use a local oscillator to convert the incoming signal to a frequency at which it is easier to obtain selectivity and gain than at the carrier frequency. In the superheterodyne method, the local oscillator frequency differs from that of the incoming signal by an amount equal to an intermediate frequency (IF); in the direct conversion method, the oscillator frequency equals the carrier frequency, which places the IF at baseband, or "zero-frequency."
After conversion, FM receivers usually amplify and limit the IF signal and demodulate it with a frequency discriminator or modulation tracking phase locked loop (PLL). A direct conversion receiver may demodulate at baseband or may retranslate the signal to a second IF for conventional demodulation. See, for example, Dual Conversion FM Receiver using Phase Locked Direct Conversion IF, U.S. Pat. No. 4,653,117, issued Mar. 24, 1987.
Closely-spaced channels require receivers to have narrow bandpass IF filters to reject adjacent channel interference and to obtain good sensitivity. Frequency tolerances in both the local oscillator and the received signal can allow the generated IF signal to fall outside the passband of the narrow filter. At UHF frequencies of 800 MHz and above, an error of only 2 parts-per-million (ppm) in the transmitted frequency could cause .+-.1.6 kHz error. The local oscillator could have similar tolerance and result in several kilohertz net frequency error at the IF. With the filter bandwidths required for 12.5 kHz channel spacing, errors of several kilohertz make it impractical to center the received signal in the IF filter passband without using additional frequency control means.
Among the prior art frequency control methods are automatic frequency control (AFC) and phase lock. AFC uses feedback from a frequency sensitive detector, such as a discriminator, to adjust the local oscillator to minimize IF frequency error at the detector caused by static offsets or slow variations in the frequency of the received carrier. Imperfections in the frequency detector or DC offsets in the control loop allow minor IF frequency errors, but these may be tolerable for many applications. Phase lock control uses feedback from a phase sensitive detector, which compares the IF signal with a stable reference and drives the local oscillator, to produce a constant IF difference frequency from the LO mixing with the received signal.
In tracking variations of the carrier frequency, local oscillator control loops interfere with demodulation of low-frequency FM information, which resides in instantaneous variations of the carrier frequency. Demodulation circuits expect these variations to be present after the carrier is translated to the IF, but the control loops remove modulation components having frequencies within their loop bandwidths. The severity of this problem depends on the frequency content of the modulation signals and on the bandwidth of the control loop. Typically, FM radio communications systems avoid modulation at DC, which requires shifts in the carrier frequency that are indistinguishable from static frequency errors. However, some signals, such as digital signalling data, may have modulation components at 10 Hertz or less.
The bandwidth required of the control loop depends on the speed with which the receiver must tune to a received signal and convert it to the IF. For example, the receiver may use a synthesized local oscillator to scan channels. Upon command, the receiver will tune its local oscillator to the frequency required to convert a received signal on a particular channel into the IF. The synthesizer produces approximately the correct frequency; then, the control loop further adjusts the LO to center the IF signal. This process may take place in less than a few milliseconds and require control loop bandwidth of more than 200 Hz. AFC systems are generally slower, but even they may be required to correct within fractions of a second when the received signal has a short duration.
Thus, a problem arises in attempting to recover low frequency FM components in receivers using wide bandwidth tracking loops. Wide loop bandwidths are desirable for rapid signal acquisition but interfere with demodulation of low frequency FM components, such as those produced by digital signalling data. It has been impractical to obtain lower frequency demodulation by decreasing loop bandwidth, because doing so reduces the ability of the radio to rapidly tune to channel.